1. Field of the Invention
The present invention relates in general to the field of information processing, and more specifically to a delta-sigma modulator-pulse-width modulator combination system and method for modifying delta sigma modulator quantizer output signals to spread harmonic frequencies of pulse width modulator output signals.
2. Description of the Related Art
Delta-sigma modulators (noise shapers) are particularly useful in digital-to-analog and analog-to-digital converters (respectively “DACs” and “ADCs”). Using oversampling, a delta-sigma modulator spreads quantization noise energy across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, a delta sigma modulator performs noise shaping by acting as a lowpass filter to the input signal and a highpass filter to the quantization noise, thus, shifting most of the quantization noise energy out of the signal band.
Delta sigma modulators can be combined with a pulse width modulator (PWM) to implement a signal processing system data converter that converts an oversampled delta sigma modulator input signal into a directly corresponding pulse width modulated output signal. The pulse width modulated output signal can be used to, for example, drive a digital amplifier.
U.S. Pat. No. 5,815,102 entitled “Delta Sigma PWM DAC to Reduce Switching” to John Melanson, granted Sep. 29, 1998 (Melanson I), U.S. Pat. No. 6,150,969 entitled “Correction of Nonlinear Output Distortion in a Delta Sigma DAC” to John Melanson, granted on Nov. 21, 2000 (Melanson II), and U.S. Pat. No. 6,480,129 entitled “Methods and Apparatus for Correction of Higher Order Delta Sigma Converters” to John Melanson, granted on Nov. 12, 2002 (Melanson III), disclose exemplary ways for implementing delta-sigma modulator/PWM combinations and are all hereby incorporated by reference.
Referring to FIG. 1, signal processing system 100 represents one utilization of a delta sigma modulator 102—PWM 104 combination. Delta sigma modulator 102 receives a digital input signal x(n) having an oversampling frequency of fOS. The oversampling frequency fOS also represents the operating frequency of the delta sigma modulator 102. “(n)” represents a particular sample of the referenced signal, and the referenced signal generally includes many samples. The data represented by input signal x(n) is originally sampled at sampling frequency fS. Thus, an oversampling ratio (OSR) of delta-sigma modulator 102 equals fOS/fS. Delta sigma modulator 102 provides a noise shaped quantizer output signal q(n) to PWM 104. Each quantizer output signal q(n) is a digital data stream representing one of N different quantization levels. In at least one embodiment, the number of different quantization levels N equals the OSR+1, where N is a positive integer. For example, the nth sample of quantizer output signal q(n) represents the nth sample of input signal x(n) with one of N different quantization levels, i.e. q(n)ε{0, 1, . . . , N−1}. The quantizer output signal q(n) is the input signal to PWM 104.
The PWM output signal y(n) is a series of frames with each frame having a period T. In each frame, PWM 104 generates a respective PWM pattern. Each PWM pattern represents a quantization level of quantizer output signal q(n). The period T of each PWM output signal frame equals 1/fOS. For each frame of a PWM output signal y(n), the duty cycle of the PWM output signal equals the pulse width duration divided by the period T. Additionally, each frame of PWM output signal y(n) can be divided into fOS/fS (i.e. N−1) discrete time slots. The time slots are defined by a quanta of time and by their position in time relative to the start of the associated PWM pattern. Each time slot tFi can be coded with a logical “1” or a logical “0”, where the number of logical 1's in a frame defines the pulse width of the frame of PWM output signal y(n) and F is the frame number and I is the time slot within the frame. The “pulse starting time” of each pulse in a PWM pattern refers to the time when the rising edge of the pulse occurs.
FIG. 2 depicts PWM patterns 200 for representative quantization levels in Table 1. Table 1 represents one example of the PWM patterns for N=65 quantization levels. The PWM output signals of Table 1 represent “centered, grow from the right” patterns with the odd quantization levels (1, 3, 5, . . . ) having right centered pulses and the even quantization levels (2, 4, . . . ) having centered pulses. Other patterns, such as grow from the left or non-centered pulses, can also be used to represent quantization levels. The PWM 104 selects a PWM pattern for each received sample of quantizer output signal q(n). Thus, each PWM pattern has a one-to-one (1:1) association with a quantization level as depicted in Table 1. Quantization levels 29-35 represent exemplary low level signals. In Table 1, “Leading Zeros” represents the number of logical “zeros” at the beginning of a PWM pattern. “Ones” represent the duration of logical “ones” in the PWM pattern following the Leading Zeros, and thus, the Ones represent the “pulse” in the PWM pattern. The total number of logical ones times the duration of each time slot in a frame of PWM output signal y(n) equals the duration of the pulse width. “Trailing Zeros” represent the number of logical “zeros” after the pulse in the PWM pattern.
TABLE 1QuantizationPWM PatternsLevelLeading ZerosOnesTrailing Zeros032032132131231231331330. . .. . .. . .. . .29182917301730173117311632163216331633153416341535153514. . .. . .. . .. . .621621631630640640
Referring to FIG. 1, the PWM output signal y(n) drives amplifier 106. The amplifier 106 in turn drives a load 108 represented by impedance Z. Load 108 is, for example, one or more audio speakers or a servo motor. In at least one embodiment, the amplifier 106 includes switches 112 and 114 that change conductivity in conjunction with the pulses of PWM output signal y(n). In some applications, amplifier 106 represents a power amplifier and has a relatively high maximum voltage, such as +30 V, and high maximum current, such as 5 A. Various factors, such as the parasitic capacitances 116 and 118 and parasitic inductance 120, cause the amplifier 106 to radiate energy at the switching frequency of switches 112 and 114 and at other harmonic frequencies of the switching frequency. It is difficult to shield this radiated energy and, thereby, prevent leakage into the rest of the system 100. Additionally, the resulting electromagnetic interference (EMI) from the radiated energy can easily exceed local EMI standards.
Audio input signals x(n) that cause pure or approximate square wave PWM patterns in frames of PWM output signal y(n) present particularly acute, potential EMI problems with system 100. Delta sigma modulator 102 quantizes each low level input signal sample as alternating or approximately alternating high and low levels. Thus, low level signals (such as pauses or silence represented by, for example, quantization levels 29-35 in Table 1 and FIG. 2) are the most common cause for the generation of pure or approximate square wave PWM patterns by PWM 104. As depicted in Table 1 and FIG. 2, quantization levels 29-35 each cause the PWM 104 to generate a frame of pure or approximate square waves. Square waves and approximate square waves present particularly problematic EMI problems because the switching frequency of switches 112 and 114 approximately equals the oversampling frequency fOS, and the oversampling frequency fOS and/or harmonics of the oversampling frequency fOS often reside in the radio frequency spectrum. For example, when fOS=384 kHZ, system 100 radiates energy at 384 kHz and at harmonic frequencies of 384 kHz. The radiated energy in the spectrum of harmonic frequencies of PWM output signal y(n) can be significant and exceed local EMI standards.
FIG. 3 depicts a frequency analysis 300 of the harmonic frequencies of 384 MHz square waves in repetitive frames of PWM output signal y(n) between 0 and 6 MHz and energy levels between 0 and −80 dB. The square waves have significant energy at the fundamental harmonic frequency of 384 kHz and significant energy at higher harmonic frequencies in the radio frequency (RF) spectrum. For example, the amplitudes of the 3rd, 5th, and 7th harmonics are respectively −12 dB, −16 dB, and −19 dB.